1. Field of the Invention
The present invention relates to a magnetic memory device.
2. Description of the Related Art
A magnetic random access memory (hereinafter, referred to as MRAM) is a general term indicating a solid-state memory in which the recorded information can be rewritten, stored and read out utilizing the direction of magnetization of a ferromagnetic material as an information recording carrier. The memory cell of the MRAM normally has a stacked structure of a plurality of ferromagnetic layers.
The information is recorded in accordance with whether the relative positions of magnetization of a plurality of ferromagnetic layers constituting the memory cell are parallel or not parallel which correspond to “1” or “0”, respectively, in binary information.
The information is written by inverting the direction of magnetization of the ferromagnetic members of the cell by the magnetic field generated by the current supplied to the write lines arranged in cross stripes.
The MRAM basically consumes no power in the storing mode of information, and is a nonvolatile memory in which information can be stored even when the power is switched off.
The information is read out by the use of the phenomenon of what is called the magneto-resistance effect in which the electric resistance of the memory cell changes in accordance with the relative angle between the direction of magnetization of the ferromagnetic layers constituting the cell and the sense current or the relative angles of magnetization between a plurality of ferromagnetic layers.
The functions of the MRAM, as compared with the functions of the conventional semiconductor memory using a dielectric material, have the following advantages:
(1) MRAM is completely nonvolatile and rewritable at least 1015 times.
(2) Nondestructive read-out operation is possible and no refresh operation is required, thereby allowing the shortening of the read cycles.
(3) As compared with the charge storage-type memory cell, the resistance to radiation is high.
The packing density, the write time and the read-out time per unit area of MRAM are estimated to be generally the same as those for DRAM. Taking advantage of the great feature of nonvolatility, therefore, applications of MRAM are expected as an external storing device for portable equipment, a memory-mixed-LSI and a main memory of the personal computer.
The MRAM now under study for commercialization includes a device exhibiting the tunnel magneto-resistance (hereinafter referred to as TMR) effect as a memory cell (See “ISSCC 2000 Digest Paper TA7.2”, for example).
The device exhibiting the TMR effect (hereinafter, referred to as the TRM device) is mainly configured of three layers including a ferromagnetic layer, an insulating layer and a ferromagnetic layer, and the current flows through the insulating layer. The tunnel resistance value changes in proportion to the cosine of the relative angle of magnetization between the two ferromagnetic metal layers and assumes a local maximum value in the case where the two magnetization are not parallel to each other.
In the NiFe/Co/Al2O3/Co/NiFe tunnel coupling, for example, the magneto-resistance change rate exceeding 25% is found in the low magnetic field of not more than 50 [Oe] (See “IEEE Trans. Mag., 33,3553 (1997)”, for example).
Known structures of the TMR device include what is called a spin valve structure in which a counter ferromagnetic member is arranged adjacent to one ferromagnetic member to fix the direction of magnetization to improve the magnetic field sensitivity (See “Jpn. J. Appl. Phys., 36, L200 (1997),” for example), and a structure having double tunnel barriers to improve the bias dependency of the magneto-resistance change rate (See “Jpn. J. Appl. Phys. 36, L1380 (1997),” for example).
Several problems have yet to be solved, however, to develop a MRAM having a packing density of not less than the order of Gigabits (Gb).
One of the problems is the reduction in the write current. In the conventional MRAM proposed, a current is supplied to the wiring and the magnetic field generated thereby is used to invert the magnetization of the recording layer of MTJ (magnetic tunneling junction). The strength of the magnetic field generated from the wiring changes with the current value of the wiring and the distance between the wiring and MTJ. In the past reported cases, however, the strength of the magnetic field generated is about several [Oe/mA].
Further, the threshold value of magnetization inversion of the recording layer of the TMR device (hereinafter, defined as the switching magnetic field Hsw) increases in inverse proportion to the size of the direction of the hard axis of magnetization of the TMR device (hereinafter, referred to as the cell width W) as shown by the equation below.Hsw=HswO+A/W 
The value A known in the prior art is 10 to 20 [Oe·μm].
The electromigration is a limiting factor against the reliability of the wiring. The electromigration is accelerated by the wiring current density. The upper limit of the current density of the Al—Cu wiring and the Cu wiring now in use for LSI fabrication is about 10 [mA/μm2] and 100 [mA/μm2], respectively.
Consider the fabrication under 0.1 μm rule required for realizing the packing density of Gb order, for example. Even in the case where the Cu wiring is used, the upper limit of the current value that can be supplied in the wiring is about 1 mA, which generates a magnetic field of about several [Oe]. In other words, in order to obtain the MRAM of Gb order, the switching magnetic field of the TMR device is required to be reduced to several tens to several [Oe].
When using the MRAM with such a reduced switching magnetic field, however, careful attention must be paid to avoid a writing error due to external magnetic fields. In mounting the MRAM on an electronic device, for example, it is necessary to consider the magnetic field leaking from the motor, the iron core of the speaker or the permanent magnet, the magnetic field leaking from the hollow core coil of the CRT or the like and the magnetic field leaking from the magnet clip used for the case open/close portion, etc. Also in other life spaces, the magnetic field leaking from the magnet clip may cause the writing error or destroy the data.
FIGS. 1 and 2 schematically show the lines of magnetic flux leaking from the permanent magnet and the hollow coil. The survey conducted by the present inventors shows that the magnetic field along the moving radius of the magnet at a position 5 mm horizontally away from the center of a cylindrical ferrite magnet (surface magnetic pole 1300 kG) 5 mm in radius and 2 mm in thickness is about 30 [Oe].
Generally, household appliances have many magnetic field sources as described above. In using the magnetic memory for these household appliances, therefore, a shield structure is required which protects the recorded magnetic information against the disturbing magnetic fields originating from the environment.
A magnetic shield structure conventionally proposed is shown in FIG. 3, for example. In this example, a magnetic memory is arranged in a hermetically sealed package magnetically shielded with a high-permeability soft magnetic material such as permalloy (See “U.S. Pat. No. 5,939,772, for example).
A package structure configuring a hermetically sealed space using a magnetic shield material, however, makes a bulky package and is undesirable from the viewpoints of both cost and the packaging technique. Especially, the use of such a package structure for household appliances poses a problem.